Many metal and polymeric composite surfaces require the protection of a coating. For example, Navy aircraft surfaces need protection from the harsh aircraft-carrier environment, which contains corrosive sea water spray and, on non-nuclear carriers, sulfur dioxide stack gases. Therefore, these aircraft need a coating which will be corrosion inhibitive as well as chemically resistant, weather resistant, flexible, even at low temperatures, and have good surface adhesion.
Currently, Navy aircraft are painted with a high performance multi-component protective paint system consisting of an epoxy primer and a polyurethane topcoat. Several types of aircraft also require a coat of spray sealant between the primer and the topcoat. More than one component has been used because no single component provides all the desirable properties.
The epoxy primer of the current system is applied to a dry film thickness ranging from 0.006 to 0.0009 inches and is designed to adhere to the substrate, promote adhesion of the topcoat, and prevent corrosion. However, the primer must be topcoated because it lacks flexibility, especially at low temperatures (-60.degree. F.), resulting in extensive cracking of the paint system in highly flexed areas of the aircraft. The primer also lacks weather resistance and cannot be formulated in the appropriate colors for an aircraft topcoat. In addition, it includes strontium chromate as a corrosion inhibitor, a known carcinogen.
The polyurethane topcoat currently used is normally applied over the primer to a dry film thickness of 0.0020 to 0.0022 inches. It provides chemical and weather resistance, flexibility, and the desired optical properties. The topcoat requires the use of a primer, however, because it will not inhibit corrosion and does not have adequate adhesion to the metal substrate.
A 0.0120 inch thick sealant coat is occasionally applied between the primer and topcoat to enhance the flexibility of the coating system. The sealant is soft and easily deformed and is difficult to apply and remove.
The current multi-component coating system described above has several deficiencies. It has a total dry film thickness ranging from 0.0026 to 0.0151 inches, adding considerable weight to an aircraft, for instance. In addition, it is time consuming to apply and remove. The system also generates a high level of volatile organic component (VOC) emissions during the painting operation.